In X-ray radiography, an automatic exposure control device (phototimer) is widely used. The phototimer is placed behind or in front of a film to convert X-rays transmitted through an object to be radiographed into an electrical signal, and shuts off X-rays when the integral of this electrical signal reaches a predetermined value, thereby keeping the density of an X-ray image constant.
FIG. 13 shows the arrangement of a conventional phototimer.
As shown in FIG. 13, the phototimer is constituted by a phosphor 108 and photomultiplier 112. The phototimer converts X-rays 102 transmitted through a human body 103 into light by using the phosphor 108, guides the light to the photomultiplier 112 through a lightguide 111, and converts the light, converted by the photomultiplier 112, into an electrical signal.
The electrical signal output from the photomultiplier 112 in this manner is stored in an integrating capacitor 113. The voltage generated by the integrating capacitor 113 when the electrical signal is stored is monitored by a processing circuit 114 constituted by a comparator 115 and density setting device 116.
When the voltage reaches a predetermined value, the processing circuit 114 sends a signal for shutting off the X-rays 102 (X-ray source control signal 117) to an X-ray source 101.
In general, one or a plurality of phototimers based on the above principle are mounted in the X-ray imaging apparatus to control the X-rays 102.
Recently, a digital X-ray imaging apparatus has been put into practice (see, e.g., U.S. Pat. No. 5,448,613), which is a combination of a two-dimensional sensor formed by depositing amorphous silicon on a glass substrate, a photoelectric conversion apparatus to which a driver circuit which drives transistors and amplifiers for amplifying signals from the two-dimensional sensor are connected, and a phosphor.
This digital X-ray imaging apparatus is designed to convert X-rays transmitted through a human body into an electrical signal, store an image as digital information, and output it. The performance of the digital X-ray imaging apparatus is equal to or higher than that of a conventional film type X-ray imaging apparatus.
The above digital X-ray imaging apparatus is becoming popular in the medical field because of its advantages, that are unnecessity of developing, ease of image processing, and ease of storage and transfer of data.
For exposure control in such a digital X-ray imaging apparatus, a phototimer like that used in a film type X-ray imaging apparatus is used.
A phototimer, however, roughly has two problems: one based on the sensitivity difference between a means for obtaining an X-ray image and the phototimer, and the other based on the number of phototimers to be installed.
In a conventional phototimer, light from a phosphor is converted into an electrical signal by a photomultiplier. Even if, therefore, the same phosphor as that used for X-ray radiography is used, the principle of the sensitivity of a film or the photoelectric conversion apparatus differs from that of the phototimer.
Conventionally, therefore, the characteristics of the phototimer are electrically corrected to become identical to those of the X-ray imaging apparatus, thereby compensating for the sensitivity difference. It is, however, difficult to precisely match the sensitivity characteristics (tube voltage characteristics and linearity), signal-to-noise ratio (S/N ratio), and offset amount of the phototimer with those of the X-ray imaging apparatus. Consequently, accurate exposure precision cannot be obtained.
In order to perform accurate exposure at any region, phototimers must be installed at positions corresponding to the respective regions. In consideration of installation places and cost, it is not realistic to install phototimers at positions corresponding to various regions.
Assume that phototimers are arranged for chest radiography alone. Even in this case, since there are physical differences between adults and children, and different densities are required depending on diagnostic regions, exposure control cannot always be relied on the phototimers.
Owing to the above problems, according to the conventional techniques, even if an X-ray imaging apparatus is equipped with phototimers, radiography cannot be relied on the phototimers in all radiographic conditions. The real situation is that radiography with proper exposure needs to be relied on the experiences of technicians regardless of whether a film type apparatus or digital apparatus is used.
In order to reduce failures in radiography, therefore, a patient is irradiated with a relatively high dose of X-rays to increase the contrast, resulting in an increase in the radiation dose of the patient.
In order to make a conventional phototimer comply with an X-ray imaging apparatus having a new principle or mechanism such as a digital X-ray imaging apparatus, it is necessary to match the sensitivity characteristics of the phototimer with those of a sensor used in the X-ray imaging apparatus. This requires many verifications.
At an early stage of proliferation of digital X-ray imaging apparatuses on the market, only a small amount of clinical data has been obtained, leading to poor exposure precision. These problems have interfered with the development and widespread use of X-ray imaging apparatuses.